Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/10634#discussion_r49656738
--- Diff:
sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java
---
@@ -125,7 +125,8 @@ public UnsafeKVExternalSorter(
// reset the map, so we can re-use it to insert new records. the
inMemSorter will not used
// anymore, so the underline array could be used by map again.
- map.reset();
+ final long spillSize = map.reset();
+ taskContext.taskMetrics().incMemoryBytesSpilled(spillSize);
--- End diff --
According to my understanding of spill metrics, a spill needs to update
both `memoryBytesSpilled` and `diskBytesSpilled`. The memory spill is the
in-memory size of the data being spilled, while the disk spill records the size
of that data after it has been serialized and written to disk. As a result, I
think that there must be a corresponding `incDiskBytesSpilled` call somewhere.
I'm thinking that this memory spill metric should be updated closer to the site
of where we increment the disk bytes spilled rather than here, since I think
doing it that way would make it easier to reason about whether we're
double-counting.
If this does turn out to be the right place for this spill, it would be
great to add a code comment explaining the rationale for why this call must be
here.
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